In a conventional hands-free telephone conversation apparatus, a sound received by a microphone installed in a vehicle is transmitted to the far-end user, and a sound from the far-end user is outputted from a loudspeaker installed in the vehicle. The sound outputted by the loudspeaker is received by the microphone as an echo when a user talks with a far-end user on a mobile phone without having the mobile phone in his/her hand. In order to cancel an echo by adding an echo cancellation signal, to a sound signal to be transmitted to the far-end user, generated through delay processing and level adjust processing, the conventional hands-free telephone conversation apparatus adjusts an operation condition of a sound processing unit for processing a sound signal to be transmitted to the far-end user and a sound signal received from the far-end user, on the basis of the volume of the sound signal to be transmitted to the far-end user, the volume of the sound signal received from the far-end user, the type of the vehicle, and the like (see for example Patent document 1).
Patent document 1: Japanese Patent Laid-Open Publication 2001-119470